This invention relates to mechanical components having improved strength and wear resistance, and is described below with particular reference to oil and gas field equipment, although being of more general applicability.
Oil and gas production equipment is for the most part made from relatively low cost stainless or carbon steel alloys. In many instances the components concerned are required to function safely in hydrogen sulphide bearing or other corrosive environments such as those containing CO2 or acids, hence the components must meet corrosion resistance standards such as National Association of Corrosion Engineers (NACE) standard MR-01-75. For alloy steels this limits the material hardness to a maximum of 22 Rockwell C, which in turn limits the maximum material strengths in tensile, yield and bearing.
In critical areas such as tubing hanger load shoulders and grooves, applied loads can exceed the capability of the restricted hardness steel. Here it has been the practice to use higher strength alloys (for example nickel rich steels such as Inconel (RTM) 718 or K Monel (RTM) 500) which are also very costly. In order to reduce material costs somewhat it is known to use higher strength (e.g. nickel alloy) inserts to distribute high load stresses from a small surface area at, for example, a load shoulder, to a larger surface area on a lower strength component, thereby avoiding the need to form the entire component from costly high strength material.
A further problem arises in critical seal bores such as for wireline plugs, pressure vessel connections and in production bore seal sleeves: that is in providing sufficient wear and corrosion resistance. Corrosion resistant base materials such as F6NM martensitic or Super Duplex stainless steels are often used at such locations, or the sealing surfaces are weld overlaid with 316SS or Inconel (RTM) 625 alloys. Whilst providing improved corrosion resistance, such materials and overlays are relatively soft and therefore prone to wear damage from erosive flows, mechanical loads and scoring.
We have realised that both the problem of improving wear resistance and the problem of improving the strength of a steel component can be met by augmenting the component with compatible higher strength materials.
Accordingly the present invention provides a steel component having improved strength and wear resistance achieved by adhering to the component a material of higher strength compatible metal alloy. The higher strength material is preferably deposited by welding, although other deposition techniques such as plasma spraying, dip coating or electro-plating can be used. If necessary the component thus formed may be heat treated to remove residual stresses and soften the Heat Affected Zone (HAZ) to NACE allowable values. The deposited material may be selected such that it is hardened by the heat treatment. For example the deposited material may be a precipitation hardenable alloy. A possible deposit material is Inconel (RTM) 725, available in welding wire form from Inco Europe Limited of 5th Floor, Windsor House, 50 Victoria Street, London SW1H 0XB. Possible component base materials are AISI 4130, AISI 8630 Mod 3 and ASTM-A182 F6NM steels. (AISI=American Iron and Steel Institute; ASTM=American Society for Testing of Materials).
The invention also provides corresponding methods of improving the strength and wear resistance of steel components.
Further preferred features of the invention are in the dependent claims or will be apparent from the following description of illustrative examples and embodiments.
With the increasing trend towards high pressure, deep well completions and resulting highly loaded support and retention shoulders, the strength of the parent material of the wellhead components is often insufficient to meet the required design criteria. This is particularly so for multi-bowl, hanger stacking and latch groove applications within wellheads. In this context the invention enables the use of low cost alloys for the wellhead component base material by increasing the strength of the load shoulders and lockdown grooves in these highly stressed areas. This may be achieved cost effectively using a deposit of high strength Inconel (RTM) 725 or other compatible precipitation hardenable alloy.
The deposit may be used for sealing surfaces, where its hardness properties afford increased resistance to abrasion and scoring in service. This is particularly advantageous where multiple make and break operations are carried out as in riser connections or in areas on components which cannot be made intrinsically scratch and wear resistant due to design related constraints.
The deposition may be carried out using any suitable conventional process, such as TIG, MIG or SMAW welding, plasma spraying, dip coating or plating processes, using hand-held or automatic equipment, as best suited to the particular application. Post weld or post deposition heat treatment may then be carried out, if applicable. This relieves any stresses built up in the HAZ and reduces the hardness in this area to within NACE allowable values. This stress relieving process may also age the deposited material overlay, increasing its strength. After heat treatment, any necessary finishing operations such as machining, grinding and polishing are then carried out. The following examples illustrate the results of test studies performed in order to determine acceptable processes for the localised deposition of Inconel (RTM) 725 onto AISI 4130, AISI 8630 Mod 3 and ASTM-A182 F6NM steels, by welding.